Optically active surface textures are typically employed in applications where surface reflections must be minimized in either the infrared (IR) or visible wavelength regions. The optical coatings with such surface textures generally comprise a dense array of microscopic features, which exhibit little or no diffraction or scattering of the incident light in the designed wavelength region. Generally, the dimensions and spacing of the microscopic features are smaller than the shortest wavelength of incident light in a particular wavelength region. In ideal cases, they should further provide a gradual transition in the effective index of refraction, i.e. a graded index of refraction from the textured surface to the ambient medium. Such microstructures are often referred to as having “moth-eye” surfaces because it has been observed that the eyes of moths reflect almost no light due to the presence of a microscopic texture.
The aspect ratio of the microscopic features is preferably high in order to provide minimal reflection at high incidence angles. In one known microstructure, an aspect ratio of greater than 3 to 1 results in reduced surface reflection and increased transmission through an optical component at incidence angles greater than 75 degrees. Unfortunately, fabrication of such microstructures is relatively complicated, and less complicated fabrication techniques such as standard micro-lithography have been incapable of producing the high aspect ratio microstructures. Additionally, known fabrication methods generally do not provide for a graded index of refraction, which is often required to provide minimal reflection for the incident light.
For example, U.S. Pat. No. 5,334,342 to Harker et al., the contents of which are incorporated herein by reference in their entirety, discloses a method of fabricating a diamond moth-eye surface wherein a polycrystalline diamond thin film is deposited on a substrate using micro-lithographic techniques. Generally, the moth-eye geometry is formed on relatively thick substrate materials rather than on a thin coating, and the geometry is formed on the substrate using a series of micro-lithographic patterning techniques, which may include dry and/or wet etching techniques. The diamond coating is then bonded to a substrate, preferably using a glass layer such as a low-temperature, refractive index-matched Chalcogenide glass.
Further, U.S. Pat. No. 5,629,074 to Klocek et al. discloses a method of embossing a pattern into a sheet of polymeric material, however, the pattern is created using a mold that is formed using a conventional diamond lathe. Unfortunately, the conventional diamond lathe cannot produce relatively high aspect ratio patterns, and as a result, low reflectivity at high incidence angles may not be possible.
Accordingly, there remains a need in the art for a relatively low cost process of forming high aspect ratio moth-eye microstructures using standard etching procedures. The process should further be capable of producing high aspect ratio moth-eye microstructures that provide a graded index of refraction and low reflectivity at high incidence angles in both the visible and infrared wavelength regions.